Increased semiconductor complexity and increased miniaturization in electronic products causes the available surface space on a printed circuit board to be at a premium. Complex circuitry requires greater numbers of signal traces while the allotted space within which those traces may occupy continues to shrink. Within the context of edge connection systems, the space along the length of a printed circuit board is also at a premium. This has led to the use of multiple rows of interconnects to a printed circuit board on closely spaced pads, for example 0.025 inch (0.635 mm) centers. As digital system operating frequencies increase, the acceptable variations in timing from one trace to another decrease. It is therefore increasingly important to minimize timing skew by minimizing the absolute trace length and the relative variations in trace length in a connector. U.S. Pat. No. 4,861,272 to Clark discloses an interconnection system for mating a multiple row connector to multiple rows of connection pads on a printed circuit board while maintaining substantially similar connector trace lengths and trace impedances.
The greater the trace densities, the greater the detrimental effects of capacitive coupling. The capacitive coupling problem is addressed in the Clark patent by minimizing the effect of high trace densities by decoupling adjacent parallel conductive trace segments. The connector interface discussed by the Clark patent effects the interconnection to multiple rows of connection pads on the surface of a printed circuit board through use of a single layer of flexible film containing coplanar conductive traces. The pad spacing therefore remains constrained by the maximum allowable trace density in a single layer of flexible film.
Closely spaced pads require an accurate and reliable connection method. Soldering produces both a high integrity electrical connection and a mechanical connection that serves to secure the connector to the printed circuit board. It is, however, increasingly difficult when soldering pads on closely spaced centers to avoid bridging (shorting two adjacent pads) and nonconnection(open circuit between the connector and any single pad). When proper connection has been made, soldering is permanent in that removal of the soldered connector to replace, repair, or retrofit a part is labor intensive, costly, and can damage the printed circuit board and the circuitry thereon.
As connectors are miniaturized and achieve higher densities, the space within which a solder joint can be made shrinks. When a rigid solder joint exists between two parts that are moveable in opposite directions, the integrity of the solder joint is threatened. The smaller the joint, the less force it can withstand. In the context of a connector with a nonzero insertion force, it is desirable to provide some form of strain relief to assure the integrity of the solder joints over repeated matings.